Service manual

1541 CIRCUIT THEORY
The Power Supply
The input AC voltage is controlled by switch 1 (SW1). Disk circuit protection is provided by fuse 1
(F1). If SW1 is closed, the AC voltage input is applied to the primary winding of transformer one (T1).
T1 steps down th AC input voltage into two smaller AC voltages. The top secondary AC output
(approx. 16VRMS) is converted to DC by the Full Wave Bridge Rectifier CR1. The DC output of CR1
is regulated at 12VDC by VR1. The bottom secondary AC output of T1 (approx. 9VRMS) is converted
to DC by the Full Wave Rectifier CR3. The DC output of CR3 is regulated at +5VDC by VR2. High
frequency filtering is provided by C1 and C3 for the 12VDC supply, and C4, C6 to C9, C22, C27 to
C30 for the 5VDC supply. Low frequency filtering is provided by C17 and C2 for the 12VDC supply,
and C5 and C16 for the 5VDC supply.
All circuit diagrams have been taken from the short board schematic 1540049 unless otherwise
noted. The short board use a 6116 RAM which replaces the four 2114 I.C.s on the long board. See
Read/Write logic differences.
The Reset Circuit
The output of the exclusive 'or' gate UD3 pin 6 will be "low" until C46 has charged through R25. Once
the voltage across C46 reaches 2 volts, the output of UD3 pin 6 will go "high". This occurs when the
disk is powered on, or a reset pulse is generated by a device connected to the serial bus. The reset
pulse on the serial bus interface is input on, pin 6 of P2 or P3. This "low" to "high" going pulse on pin
6 of UD3 is input to the microprocessors reset interrupt input. This causes a restart on reset routine to
be executed giving control of the disk drive operation to the Disk Operating System (DOS).